Generally, many materials need to be purified. Purification is also required to organic materials and essential amino acids used for feedstuff additives, medicines, health foods, etc. For example, organic materials used for organic light-emitting device are obtained by isolating a pure target component from the conventional synthetic substance for the use purpose in thin film deposition. With enhanced purification techniques of the organic materials, the organic light-emitting devices are allowed to improve the light-emitting efficiency and extend the light-emission life span.
For the production of organic materials on a large scale, it is essential to use the purification techniques for organic materials with reduced process time and enhanced purification efficiency.
Tryptophan is one of essential amino acids. The conventional purification techniques for tryptophan are disclosed in Japanese Patent Publication Nos. 126070/1986, 39857/1984 and 126070/1983.
However, the method of using an ultrafiltration membrane according to the technique of Japanese Patent Publication No. 895/1983 cannot eliminate impurities merely at a fixed elimination rate, ending up imposing an overload on the resin and reducing the reuse cycle of the non-polar highly porous resin.
Further, the method specified in Japanese Patent Publication No. 39857/1984 is insufficient in eliminating impurities and makes it difficult to obtain crystals with transmission of 95% or higher.
Furthermore, the method disclosed in Japanese Patent Publication No. 126070/1986 forms the structure of tryptophan including unstable indole rings when heating the reaction solution containing lots of impurities and prepared by the fermentation method up to 95 to 100 C in the presence of active carbon, which increases the formation of decomposed and discolored substances.